As a countermeasure against black smoke, a diesel particulate filter (DPF) has been used to collect particulate matter (PM) in exhaust gas. Forced regeneration is performed for the DPF to periodically burn and remove the collected PM. Specifically, when an estimated PM deposition amount in the DPF reaches a predetermined amount, the temperature of exhaust gas is forcibly raised so that the PM is burned to be removed.
However, depending on an operation state of the engine, abnormal combustion of the PM might occur during the forced regeneration. As a result, the excessive temperature rise might occur in the DPF, and the DPF might be damaged by heat erosion or cracking. As control for preventing the excessive temperature rise due to the abnormal combustion in the DPF, techniques in Patent Document 1 (Japanese Patent Application Laid-open No. 2011-153591) and Patent Document 2 (Japanese Patent Application Laid-open No. 2003-206726) have been known.
Patent Document 1 discloses the following technique. Specifically, when an internal combustion engine transitions from or an operation region α in a high speed state or with a high load to an operation region β with a low speed and a low load within a set time T1, it is determined that the engine is in a DPF abnormal combustion occurring operation. When it is determined that the engine is in the DPF abnormal combustion occurring operation, an intake throttle valve 4 is fully opened so that an exhaust gas flowrate increases. The DPF filter device is cooled through heat removal with sensible heat of the exhaust gas. Furthermore, late post injection is continuously performed, so that oxygen concentration around the DPF filter is reduced, so that the abnormal combustion of the PM collected by the DPF filter is prevented. Thus, the abnormal temperature rise in the DPF filter device can be prevented in an early stage.
Patent Document 2 discloses the following technique. Specifically, when an engine transitions to a low load/speed region in which an exhaust gas flowrate of the engine decreases during the regeneration of the DPF, a burner is operated so that combustion gas is supplied into a turbine of a turbocharger. Thus, the turbine work is increased to facilitate turbocharging, and the heat dissipation of the DPF is facilitated as the exhaust gas flowrate of the engine increases, so that the excessive temperature rise is prevented.